Wet-bulb temperature

Wet-bulb temperature - there are several meanings of this term:

1. The temperature read from a wet bulb thermometer,
2. Isobaric wet-bulb temperature: the temperature an air parcel would have if cooled adiabatically to saturation at constant pressure by evaporation of water into it, all latent heat being supplied by the parcel;
3. Adiabatic wet-bulb temperature: the temperature an air parcel would have if cooled adiabatically to saturation and then compressed adiabatically to the original pressure in a moist-adiabatic process (AMS Glossary).

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Wet-bulb temperature measured by wet bulb thermometer

Wet-bulb temperature is measured using a thermometer that has its bulb wrapped in cloth—called a sock—that is kept wet with water via wicking action. Such an instrument is called, not surprisingly, a wet-bulb thermometer. At relative humidities below 100%, water evaporates from the bulb which cools the bulb below ambient temperature. To determine relative humidity, ambient temperature is measured using an ordinary thermometer, better known in this context as a dry-bulb thermometer. At any given ambient temperature, less relative humidity results in a greater difference between the dry-bulb and wet-bulb temperatures; the wet bulb is colder. The precise relative humidity is determined by finding one's wet-bulb and dry-bulb temperatures on a psychrometric chart (or via complex calculation).

where:

RH is relative humidity and T_d is dew point in degrees Celsius

T and T_w are the dry-bulb and wet-bulb temperatures respectively in degrees Celsius

e_s, e_w and e are the saturated water vapour pressure at the dry-bulb, wet-bulb at saturation pressure and normalized to sea level pressure wet-bulb (hPa)

for greater accuracy use the Arden Buck Equation to find the water vapour pressures

Hygrometers are instruments comprising both wet-bulb and dry-bulb thermometers. A hygrometer can also be used in combination with a globe thermometer (for measuring solar radiant heat) in the calculation of the wet bulb globe temperature.

See also

• wet-bulb potential temperature
• Dewpoint
• Wet Bulb Chart for Snow Making (Fahrenheit)
• http://www.srh.noaa.gov/elp/wxcalc/formulas/rhTdFromWetBulb.html
• http://amsglossary.allenpress.com/glossary/search?p=1&query=wet-bulb+temperature
• http://www.theweatherprediction.com/habyhints/170/ (Shortcut to estimate wet bulb temperature)